I assume they're taking advantage of systems that are already in place, at scale, at Facebook. Then, to ensure that this app has the privacy that people expect

I assume they're taking advantage of systems that are already in place, at scale, at Facebook. Then, to ensure that this app has the privacy that people expect from an app of the ephemeral kind, they are baking in additional precautions. In this case, a key that gets destroyed is the precaution. I am not an insider, this is just a guess, etc.